Wednesday, 11 May 2016

Mirach's Ghost

Mirach is the brightest star in the constellation of Andromeda. It has a deep red colour indicating that it's cooled down significantly as it expanded to a size roughly 100 times our Sun towards the end of its life. Its colour is so stable that it has served as one of the references for stellar classification. Apart from this, you wouldn't expect anything particular about it. It's just an old giant star without any characteristics that would make it worthwhile dedicating a sketch to it. Well, at first sight anyway.

But hoho... Wait a minute! What's that faint patch on the right of the field of view? Yes, that's right! It's a galaxy! It's not all that easy to see because bright Mirach is slightly blinding you and that's why they nicknamed it "Mirach's Ghost". Now you see it, now you don't. But when you let your eyes adjust to the field of view it will certainly leap out at you, even through not very large telescopes.

NGC404, its scientific name, is a dwarf galaxy at the reasonably close distance of 10 million lightyears. Yet, it is classified as a so-called "field galaxy" because it doesn't seem to have any gravitational interaction with other galaxies, even though it's quite near to our local group. The poor little thing just lies there, completely isolated and very inactive. Like other early-type galaxies such as M105, there's very little star formation going on and it appears to be slowly dying. Scientists believe that at some point it had a spiral structure and hence was very active but that a dramatic merger with a companion, some 1 billion years ago, reduced it to its almost vegetative state. Very few details can be discerned and especially through amateur telescopes you shouldn't expect to see more than a blurry little patch. Analysis revealed however that it has a pair of haloes of neutral hydrogen and that it houses a massive black hole too.

So all in all, the combination of two seemingly uninteresting objects, Mirach and NGC404, still make a wonderful observation. Especially since both the star and the galaxy have a lot in common: both have reached the end of their life and both display very little activity. Astronomy isn't always about spectacles, cataclysms and dazzling star fields. Sometimes, something very simple can prove to be a beautiful tableau.